No abstract

Abstract. Previously, we showed that chitin synthase 2 (Chs2) is required for septum formation in Saccharomyces cerevisiae, whereas chitin synthase 1 (Chsl) does not appear to be an essential enzyme.However, in strains carrying a disrupted CHS1 gene, frequent lysis of buds is observed. Lysis occurs after nuclear separation and appearg to result from damage to the cell wall, as indicated by osmotic stabilization and by a '~50-nm orifice at the center of the birth scar. Lysis occurs at a low pH and is prevented by buffering the medium above pH 5. A likely candidate for the lytic system is a previously described chitinase that is probably involved in cell separation. The chitinase has a very acidic pH optimum and a location in the periplasmic space that exposes it to external pH. Accordingly, allosamidin, a specific chitinase inhibitor, substantially reduced the number of lysed cells. Because the presence of Chsl in the cell abolishes lysis, it is concluded that damage to the cell wall is caused by excessive chitinase activity at acidic pH, which can normally be repaired through chitin synthesis by Chsl. The latter emerges as an auxiliary or emergency enzyme. Other experiments suggest that both Chsl and Chs2 collaborate in the repair synthesis of chitin, whereas Chsl cannot substitute for Chs2 in septum formation.

Previous work led to the puzzling conclusion that chitin synthase 1, the mnujor chitin synthase activity in Saccharomyces cerevisiae, is not required for synthesis of the chitinous primary septum. The mechanism of in vivo synthesis of chitin has now been clarified by cloning the structural gene for the newly found chitin synthase 2, a relatively minor activity in yeast. Disruption of the chitin synthase 2 gene results in the loss ofwell-defined septa and in growth arrest, establishing that the gene product is essential for both septum formation and cell division.